This invention relates to arrangements for paralleling amplifiers for increased output power, and more particularly to arrangements which combine power in a hybrid power combiner including sum and difference ports, and which control the phase of the signals being combined in response to the power level at the difference port.
The paralleling of radio-frequency or microwave amplifiers and/or signal sources for higher combined power is well known. Radio frequency (RF) signals includes microwave, millimeter wave and other signals. U.S. Pat. No. 4,701,716, issued Oct. 20, 1987 in the name of Poole describes an arrangement for paralleling traveling-wave tube (TWT) amplifiers. As described therein, the signal to be amplified is applied to a 3 dB, 90.degree. hybrid for splitting the signal into two nominally equal-amplitude portions. A similar 3 dB hybrid combiner includes a pair of input ports and a combined output port. A pair of amplifying paths extend from the output ports of the signal splitter to corresponding input ports of the power combiner. As described in the Poole patent, the phase and path length through the amplifier paths are equalized to maximize the combined power over a broad bandwidth. The difference ports of the hybrid splitters and combiner are terminated in resistors.
Paralleled amplifier arrangements may be used for transmitting radio frequency or microwave signals in repeaters and communication systems located in remote or inaccessible locations, as for example, mountain tops. The sensitivity of the phase characteristics of TWT amplifiers to power supply voltage fluctuations and to aging is notorious in the art. Solid-state amplifiers are generally not as sensitive, but may be subject to sudden changes in phase or amplitude response as a result of transient electromagnetic interference (EMI) conditions, the effects of nuclear radiation, or as a result of age-related effects due to diffusion of semiconductor dopants at high temperatures. A change in phase of less than about 10 or 15 degrees results in a reduction of combined output power of less than about 0.1 dB. Such reductions in power are ordinarily considered to be acceptable. Relative phase changes exceeding 10 or 15 degrees may result in power reduction substantially greater than 0.1 dB, or even may result in cancellation of the signal. Such changes are often considered to be unacceptable. The use of manually adjustable phase shifters in the amplifier paths is known for correcting the phase so as to achieve the desired output. When the paralleled amplifier is located in a remote or relatively inaccessible location, phase correction may require the dispatching of a maintenance technician to the location in order to correct the problem. During the interval between the occurrence of the anomaly and the arrival of the technician, the equipment may be inoperative. This may be undesirable. In some locations, as for example in submarine cables, it may not be possible to make a manual adjustment at all.
For airborne and other systems, it is important to minimize the size, weight and complexity of the equipment, including the paralleled amplifier. An improved phase correction arrangement is desired for paralleled amplifiers.